Synchronization of facsimile telegraph apparatus



0a. 27, 1931. R. H. RANGER 1,829,420

SYNCHRONIZATION OF FACSIMILE TELEGRAPH APPARATUS Filed Aug. 19, 1925 I 2Sheets-Sheet l I g] 2) F/%. 1 45 E 47 *wvw Reduction fl wn'ng l INVENTORHARD H. RANGER BY W A RNEY Get. 27, 1931.

SYNCHRONIZATION OF FACSIMILE R. H. RANGER TELEGRAPH APPARATUS Filed Aug.19, 1925 2 Sheets-Sheet 2 F s F s F s F 24 23 22 25 24 23 22 25 A 5 F 5F 5 F 5 F 24 Z3 Z2 Z6 24 Z3 Z2 25 B q r s F 5 F .2 F 2 24 23 22 '25 242x 22 2.;

INVENTOR RICHARD H. RANGER ATTORN EY Patented Oct. 27, 1931 UNITEDSTATES PATENT OFFICE RICHARD ROWLAND RANGER, 0] NEWARK, NEW JERSEY,ABSIGNOB TO RADIO COD- PORATION OI AHERICA, A CORPORATION OF DELAWARESYNCHRONIZA'I'ION OI FACSIMILE TELEGRAPH APPARATUS Application fledAanst 18, 1925. Scrial lo. 51,191.

This invention relates to certain new and useful improvements in speedregulation of electric motors and more articularl y' to a method of anapparatus or obtaining an absolutely constant and invariable s eed ofsuch a motor. While it will be un erstood that my invention is ofgeneral application and may be used in all situations 1n which extremelyconstant speed is required, it is particularly adapted to systems forthe transmission and reception offacsimile messages, such as pictures,copies of documents and the like either by radio or by wire line.

In the past various methods have been 'su gested for obtaining constantspeeds, but in general these methods have failed to give the desiredresults, lparticularly when used in apparatus in whic telegraphy inwhich the transmitting and receiving apparatus is required to besynchronized.

Many methods have been suggested in which synchronization has beenattempted by regu ation of one motor by another by means of impulsesdelivered by the first to the second. The fundamental idea of suchmethods is that the second motor is thus caused to follow the firstmotor in respect to all variations of speed.

I have found that in practice and more particularly in the transmissionof facsimile messages by radio, as distinguished from such transmissionby wire line, it is impracticable to attempt to regulate the speed ofone motor by the speed of another. Particularly, it is impracticable toattempt to adjust the speed of the motor at the receiver to the speed ofthe motor at the transmitter by means of correcting impulsesperiodically sent by the transmitter.

The effect of atmospheric disturbances, commonly called static, strays,:vs and the like issuch'that, no matter what type of impulses are usedso far as I know, these atmospheric disturbances always interfere withand upset the synchronization of transmitter and receiver according toany method requiring the transmission of synchronizing or correctingsignals.

a very great constancyv .of speed is required, such as in facsimile Itis an ob'ect of this invention to produce a system an apparatus forthetransmission of facsimile messages which is not subject todisturbance by atmospheric disturbances, statlc, or other forms ofInterference.

It is a further object of this invention to produce a system andapparatus for the transmission of facsimile messages without requiringthe transmission of s nchronizing signals between transmitter anreceiver.

' It is a further object of this invention to produce a system for thetransmission of facs1mile messages in which the transmitting and receivmapparatus is synchronized or ad usted to t e same speed independently.

It is a further object of this invention to produce a speed regulatingsystem of suflicient accuracy and sensitivity to permit independentadjustment of transmitter and receiver to the same speeds without thetrans mission of synchronizing signals.

It is a still further object of my invention to provide a speedregulating system which shall be positive and efiective and which willregulate speed to a greater constancy than systems hitherto known in theart.

Still other objects will beapparent from the specification.

Satisfactory synchronization without the transmission of synchronizingsignals is in reality a problem of speed regulation, since it is possile to obtain tuning forks,'pendulums, clocks and the like, which can beregulated very closel to the same frequency or speed. If then t etransmitter can be rigorously held to the speed determined by such ameans, transmitter and receiver ma vbe independently adjusted and heldto t e same predetermined speed.

Since I do not use any correction or control between transmitter andreceiver in operation and since it is necessa in synchronizing accordingto my invention only that the tuning forks, pendulums, clocks or thelike used for regulation have nearly the same frequency period or speed,I have shown and described my invention with reference to a single motorwhich may be used to drive a transmitter or receiver as the case may be,but it will be understood that in the transmission of pictures byapparatus embodying my invention, the same method will be used forsynchronizing at each end.

I have found that by the use of a tuning fork, for example, so chosen asto make one complete beat in approximately the time that the motornormally makes one complete revolution, it is possible to arrangecircuits which will regulate a motor to an approximately constant speedwhich may be satisfactory for certain purposes. In general, however, forthe transmission of facsimiles a still greater constancy of speed isnecessary and for this purpose I apply a correction proportional to thedeparture of the motor from the speed desired, but of relatively minoreffect or what may be termed a Vernier correction.

Briefly. it may be stated that according to this invention a rough speedadjustment or correction of the motor speed proportional to thedeparture from the desired speed is applied by the tuning fork at leastonce during every revolution of the motor which sufiices to give anapproximately constant speed and in addition to this. I apply a veruiercorrec-- tion at relatively long intervals, for example once everysecond, which, while of itself not sufficient to hold the motor to aconstant speed. is sutficient when applied as a correction to theapproximately constant speed maintained by the tuning fork.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself however, both as to its organization and method of operationtogether with further objects and advantages thereof. will best beunderstood by reference to the following description taken in connectionwith the accompanying drawings, in which Fig. 1 is a diagrammatic viewof a circuit in accordance with my invention; and

Fig. 2 is a chart illustrating the action of the speed controllingmeans.

Referring more particularly to Fig. 1, 1 designates generally anelectric motor, herein shown as of the shunt-wound type having anarmature 2 and a field winding 3. For the purpose of controlling thespeed of this motor, as will be more particularly set forth hereinafter,I provide a tuning fork herein shown as 4. It will be understood,however, that the use of a turning fork is not an essential part of myinvention butthat any other suitable or equivalent apparatus may be substituted for the tuning fork. the only requirement being that theelement 4 should be a mechanical or electrical vibrator or oscillator ofrelatively constant frequency and of the order of frequency of the motorspeed expressed in revolutions per second.

For the purpose of driving the tuning fork 4 I have shown a coil 5connected in the customary way to keep the fork in vibration,

as is well understood in the art. Specifically, the coil 5 is connectedon theonehaud through a suitable resistance shown as 7 to one side ofthe supply line indicated as and on the other hand through contact 8 andthe fork itself 4 to the other side of the line indicated as It Will beunderstood that after the fork is once set into operation, the vibrationof the tine to and away from contact 8 will cause current impulsesthrough the coil 5 which will cause the fork to continue to vibrate. Inaddition, I have shown a coil 6 connected across the source of supply incircuit with a variable resistance 14, the purpose of which is to givewhat may be termed a Vernier adjustment of the fork frequency. It willof course be understood that any suitable means may be provided forvarying the frequency of vibration of the fork 4, such for instance assliding weights mounted upon the tines or any other equivalentarrangement.

From this it will be understood that the fork 4 may bet set intovibration and maintained in vibration at a substantially constantfrequency and that the frequency may be varied within limits to theextent desired. By the operation of suitable adjustments, the fork 4, aswill hereinafter be pointed out, is arranged to control in what may betermed a coarse way the speed of the motor. This may be accomplished bythe following arrangement, for example.

Driven by the shaft of the motor 2, and preferably but not necessarilymounted thereon, I provide a brush 20. Cooperating with this brush 20 Iprovide a commutator designated generally by 21 and having, for example, four segments 22, 23, 24 and As shown, two of these are connectedtogether; for example, 22 and 24 are so connected; likewise 23 and 25.From brush 20 a connection is made to contacts 12, 13, adapted to beclosed by the vibration of the fork 4, and the fork 4 is itselfconnected to one side of the line, as previously pointed out.

In the armature circuit I provide a resistance. herein shown as 14, andin the field circuit I provide a suitable resistance 15. The commutatorsegments 23 and 25 are connected at an intermediate point betweenresistance 14 and the armature 2. In a similar way segments 22 and 24are connected at an intermediate point between field 3 and resistance15. I have also provided resistances 16 and 18 in series with condensers17 and 19 respectively across the field and armature terminals for thepurpose of smoothing out the current flowing therein, put these form noessential part of my inven- Similarly, I have shown resistance 28 inseries with condensers 26 and 27 respectively connected between brush 20and adjacent seg ments of commutator 21. These likewise --have alsoshown condensers and 11 connected between contacts 8, 12 and 13respectively and an additional resistance 46 for connecting the pointbetween'condensers 10 and 11 to the source of supply. It will beunderstood also that these orm no essential part of my invention.

It willnow be apparent that on the closure of switch S in the-motorcircuit the motor 1' will be set into operation. The tuning fork 4 maalso be set into 0 ration by tapping it and the fork will pre erably,but not necessarily, be so adjusted that its frequency is substantiallythe same as the number of revolutions per second of the motor.

As the motor shaft revolves, brush 20 will sweep around the segments ofcommutator 21 and will alternately close a circuit from the terminal ofthe armature circuit connected to resistance 14 around the resistance 14to contacts 12-and 13 of the fork and from the terminal of the fieldwinding connected to resimance 15 around the resistance 15 to the samepoint. If now fork 4 is in contact with either of contacts 12 or 13, acircuit will be completed to the positive'side of the supply line,short-circuiting alternately resistances 14' and 15.

It will readily be understood that the shortcircuiting of resistance 14'in the armature circuit Wlll cause the motor to tend to speed up and onthe other hand, short-pircuiting of resistance 15 in the field circuitwill cause the motor to tend to slow down.

Referring to section A of Fig. 2, I have shown diagrammatically theaction of the commutator 21 and brush 20 in cooperation with the fork 4.The closures of the various contacts are plotted against time. Byreferring to the first line, it will be seen that brush 20 closessuccessively a circuit through commutator segments 24, 23, 22 and 25. Itwill be apparent from what has already been said that the closure of thecircuit through segments 24 and 22 prepares a circuit for slowing downthe motor by short-circuitmg the field resistance, and on the other handthe closure of the circuit through segments 23 and 25 prepares a circuitfor speeding up the motor through the short-circuiting of armatureresistance. These closures are designated F and S, meaning faster andslower.

Referring further to section A of Fig. 2, it will be seen that throughthe alternate closure of contacts 12 and 13 with the fork 4, a circuitis periodically completed from points 12 and 13 to the positive supplyline. In case the motor and fork are operating at the same speed, thatis to say, the motor making one complete revolution while the fork makesone complete beat, it will be apparent that there will be what may betermed a speeding up impulse given to the motor twice during arevolution and what may be termed a slowing down impulse twice during arevolution. One of these speeding up immg over commutator 23 and whilecontact 13 is closed'by fork 4. One of the slowing down impulses willoccur immediately afterward while brush 20 is passing over a portion ofcommutator se ent 22 and while contact 13 is still close If the motor isoperating at the same speed as the fork and neither gaining nor losing,these s eding up and slowing down impulses will exactly equal in lengthand owin to the inertia of the armature no change In speed will occur.

If on the other hand the motor is tending to slow down, the conditionswill be as shown in section B of Fi 2; that is to say, brush 20 will lagbehind t 0 position it should take and instead of the alternate speedingup and slowing down impulses being of equal length, the speeding upimpulses will increase in length and the slowing down impulses willdecrease in length. This will of course cause the motor to tend to speedup to a point where the pulses are equal in length.

In a similar way, it will be seen that if the motor tends to speed up,conditions are as shown in section C of Fig. 2, wherein brush 20 is inadvance of the position which it should have and closes its circuitthrough the commutator segments too soon. The result of such action isto cause the time of the slowin down impulses to increase and the time othe speeding up impulses to diminish, thereby tending to slow the motorto the proper speed.

By the use of the arrangement just described, it is possible to obtainwhat may be called a rough adjustment of speed. The adjustment however,while it may be satisfactory for some purposes, does not give theextreme constancy of speed that I have found desirable for certainpurposes and for the purpose of making it still more nearly constant, Ihave provided further means for obtaining a very fine adjustment ofspeed.

For this purpose I have made use of a standard or precision clock, suchas diagrammatically illustrated at 29 as a pendulum having a contact 30.The clock per se forms no part of this invention and is therefore notdescribed in detail, it being suflicient for the purposes of thisinvention to state that there is provided a contact 30 closed forexample once every second by the clock 29.

The contact 30 is connected to one terminal of the source of supply asshown and the contact on the clock 29 is connected through suitableresistance 36 through coil 34 to the other side of the supply line. Acircuit is also established through coil 35 and suitable resistance 37through contacts 32 and 33 closed by a cam 31 driven by motor 2 andwhich will preferably be arranged to close contacts 32 and 33 at thesame time and as often as conpulses will occur while the brush 20 ispass- Ill tact is closed by clock 29. An obvious means to rotate the cam31 so as to turn same one revolution durin each swing of the clockpendulum 29, is to d rive the cam through a reduction gearing drivenfrom the motor shaft. The reduction gearing may be of any preferredratio to give the desired speed rotation to the cam. Coils 34 and 35 areso arranged as to control a suitable armature 42 and attract it againstcontact 41 or against a suitable dummy contact.

Armature 42 is connected as shown to one side of the source of supplyand is also connected through a suitable small motor having an armature39, field coil 38 and a suitable resistance 43. Motor 39 is arranged tocontrol the position of commutator 21 by means of any suitable drivingmechanism, such for instance as worm 44; that is to say, operation ofthe motor will cause a slow rotation of commutator 21.

Connected to one side of the armature 39 of this motor I provide aresistance 40 which in turn is connected to contact 41. When thearmature 42 is closed against contact 41, a circuit of suitableresistance will be established around the armature and current will bediverted from the armature. The constants are preferably chosen so thatwhen armature 42 is closed against contact 41, the motor will just notoperate and when armature 42 is attracted against the dummy contact, themotor will operate and commutator 21 will be slowly rotated.

It is to be noted that coils 34 and 35 are so arranged that the effectof coil 35 predominates and is sufficient to attract armature 42 againstcontact 41, although coil 34 may in fact be energized. It will also beunderstood that armature 42 will remain in either of the positions inwhich it happens to be until it is attracted to the other position byeither coil 34 or 35. The operation of this apparatus will now be clear.

If the motor 1 is operating at precisely the desired speed, contact 32will be closed against contact 33 by the action of cam 31 exactly asoften as contact 30 is closed by the action of clock 29. If theseclosures take place simultaneously, nothing further happens, armature 42being attracted against contact 41 by the action of coil 35, therebydiverting current from armature 39 through resistance 40. If, as may bethe case, these do not occur simultaneously, then for a certain part ofthe time armature 42 will not be attracted against 41 and the smallmotor will operate, shifting commutator 21 in a direction to cause themotor to speed up or slow down for a short time, until the closures aresimultaneous.

For the purpose of indicating whether or not the motor is operating atthe speed desired, I provide a small lamp 45 which may conveniently bemounted ofl center upon the motor shaft. While the particular type oflamp used forms no part of my invention, it is evident that one shouldbe used which has practically no thermal inertia and for this purpose Ihave found a small neon lamp to be satisfactory.

The circuit through this lamp may be closed, for example, once a secondby the clo sure of fork 4 on contact 9, thereby energizingauto-transformer 46 for a short time, once per beat of the fork. If themotor is runningin synchronism with the fork and the lamp is observed,it will be seen as a point of light standing stationary upon the motorshaft. If the motor is running slow, the point of light will appear tobe moving backwards, that is, against the direction of rotation and iffast, to be moving forward.

It will be understood that manually operable means may be provided forcarrying on the speeding up and slowing down operatlons.

herein set forth, such as manually operable switches for varyingarmature or field resistance and for operating motor 39, and alsomanually adjustable or controllable means for shifting the position ofcontacts 32 and 33 with respect to cam 31. It will also be understoodthat this position may be shifted by a suitable motor and gearing.

IVhile I have shown and described the preferred embodiments of myinvention, it will be understood that I do not wish to be limited toprecisely the form shown and that modifications and changes may be madewithout departing from the spirit of my invention.

Having described my invention, what I claim is:

1. A method of synchronizing apparatus used in the transmission andreception of messages by radio which is unaffected by static whichconsists in adjusting the driving apparatus of the transmitter andreceiver independent of connecting channels therebetween tosubstantially constant speeds at relatively frequent intervals, and inapplying secondary adjustments to the driving apparatus of thetransmitter and receiver also independently of each other at relativelywidely separated time intervals.

2. A method of synchronizing apparatus used in the transmission offacsimile messages by radio which is unaflected by static, whichconsists in controlling the speed of driving mechanism of receiver andtransmitter independently of connecting channels therebetween byseparate constant frequency vibrators for giving a coarse speed adjustment and a second set of constant frequency vibrators for giving a finalspeed adjustment.

3. A method of regulating the speed of a moving mass which comprisesapplying a correction responsive to deviation from desired speed atrelatively frequent intervals and applying at relatively long intervalswith respect to said first mentioned correctime therewith and applyingat relatively .long intervals with res ect to said first mentionedcorrection a fu er correction res onsive to deviation from desired speedintegrated over a relatively long period of time.

5. A method of regulating the speed of a moving mass which com rises aplying a correction responsive to eviation rom desired speed atrelatively frequent intervals and applying a correction in response tode viations from desired speed too small to be corrected by said firstcorrection and only at relativel lon intervals.

6. A met od 0 regulating the speed of a moving mass which comdprlsesapplying a correction responsive to eviation from desired speed atrelatively frequent intervals to maintain an approximation to desiredspeed and a plying a further correction in response to eviationintegrated over a relativel lon period of time to maintain exact d esirespeed.

7. A system for regulating the speed of a movable mass comprising, incombination, means responsive to instantaneous deviation of said massfrom desired speed for applying a correction thereto, and means operableat relatively long time intervals for apply ing a correction responsiveto deviations in desired speed integrated over said intervals.

8. A system for regulating the speed of a .movable mass comprising, 1ncombination,-

means responsive to instantaneous deviation of said mass from desiredspeed for applying a correction thereto and means operable at relativelylong time intervals responsive to deviations in desired s ed integratedover said intervals for initiatlng operation of said first named means.

9. A system for regulating the speed of a movable mass comprising, incombination, primary speed regulating means responsive to instantaneousdeviation of said mass from desired speed for applying a correctionthereto and secondary speed regulating means responsive to integrateddeviations of speed for causing operation of said primary speed Aregulating means.

10. A system for regulating the speed of a movable mass comprising, incombination, a substantially constant frequency vibrating element, meansfor controlling the speed of said mass in accordance with the frequencyof said vibrating element, a second constant frequency vibrating elementof materially lower frequency than said first vibrating element andmeans for further controlling the speed of said mass in accordance withthe speed of said second vibrating element.

11. A system for regulating the of a rotary body comprising incombination, a

substantially constant frequency vibrating element adapted to make onevibration in substantially the time required by said body to make onerevolution, a second constant frequency vibrating element adapted tomake one vibration in t e time required by said body to make acomparatively large number r of revolutions and means controlled bv bothsaid vibrating elements for controlling the speed of said body. 12. Asystem for regulating the speed of a rotary body comprising incombination, a

substantially constant frequency vibrating element adapted, to make onevibration in substantially the time required b said bod to make onerevolution, means or contro ling the speed of said body in accordancewith a the frequency of said vibrating element, a second constantfrequency vibrating element of materially lower frequency than saidfirst vibrating element and means controlled by said second vibratingelement in response to deviations of said body from desired s eed forcausing operation of said controfiing means.

13. A system for regulating the speed of a rotary body comprising incombination, a I

constantly vibrating current interrupter, a plurahty of contactsarranged to be successively closed by said body in operation and meanscontrolled conjointly by said interrupter and said contacts forperiodically ingrelasmg and decreasing the speed of said 14. A systemfor regulating the speed of a rotary body comprising in combination, a

constantly vibrating current interrupter of substantially constantperiodicity, a plurality of contacts arranged to be successively closedby said body in operation and means cooperating with said currentinterrupter and said contacts for causin said body from desired spee tointroduce a force for correcting said deviation.

15. A system for regulating the speed of a rotary body comprisin incombination, a substantially constant 1%" terrupter, a plurality ofcontacts arranged to be successively closed by said bod in operation,and circuits connectin said interrupter and said contacts for period anddecreasing the speed of-said dy by substantially equal minuteincrements.

16. A system for regulating the speed of a rotary body comprisin incombination. a substantially constant requency current interrupter, aplurality of contacts arranged to be successively closed by said body inoperation, and circuits connecting said interrupter and said contactsfor periodically increasing and decreasing the speed of said body bsubstantially equal minute increments, an such circuits being soarranged as to vary the ratio equency current in- I deviations ofbetween speed increasing and decreasing increments in response todeviations of said body from desired speed and in a direction to correctsaid deviation.

17. A system for regulating the speed of a rotar body comprising, incombination, a circuit operable to increase the speed of said body, asecond circuit operable to decrease the speed of said body, a pluralityof contacts successively operable b said body for successively preparingsai circuits at predetermined intervals and for predetermined times, anda constant frequency speed determining interrupter associated therewithand arranged to complete the operation of one or the other of saidcircuits and to vary the time interval of operation to maintain thespeed of said body constant. v

18. A system for regulating the speed of a rotary bod comprising, incombination, a circuit opera le to increase the speed of said body, asecond circuit operable to decrease the speed of said body, a pluralityof contacts successively operable by said body for successivelypreparing said circuits at predetermined intervals and for predeterminedtimes, a speed determining interrupter associated therewith and arrangedto complete the operation of one or the other of said circuits and tovary the time interval of operation to maintain the speed of said bodyconstant, a second speed determining interrupter of materially lowerspeed than the first speed determining interrupter, a second set ofcontacts operable by said body, and means controlled conjointly by saidsecond speed determining interrupter and said second set of contacts orvarying the time intervals of operation of said circuits to correct fordeviations in speed integrated over relatively long periods.

19. A system for regulating the speed of a rotary body comprising, incombination, a circuit operable to increase the speed of said body, asecond circuit operable to decrease the speed of said body, a pluralityof contacts succesively operable by said body for successivel preparingsaid circuits at re-' determine intervals and for predetermined times, aspeed determining interrupter associated therewith and arranged to comlete the operation of one or the other of sai circuits and to vary thetime intervals of o eration to maintain the speed of said b y constant,a second speed determining interrupter of materially lower speed thansaid first speed determining interrupter, a second set of contactsoperable by said body and means controlled by said second speeddetermining interrupter and said second set of contacts for varying theposition of said first mentioned plurality of contacts with respect tosaid body.

20. A system for regulating the speed of a rotary body comprising, incombination, a

circuit operable to increase the speed of said body, a second circuitoperable to decrease the speed of said body, a plurality of contactssuccessively operable b said body for successively preparing saicircuits at predetermined intervals and for predetermined times, a speeddetermining interrupter associated therewith and arranged to completethe operation of one or the other of said circuits and to vary the timeintervals of operation to maintain the speed of said body constant, asecond speed determining interrupter of materially lower speed than saidfirst speed determining interrupter, a second set of contacts operableby said body, means for shifting the position of said plurality ofcontacts with reference to said body and means op erable in response tonon-coincident operation of said second speed determining interruper andsaid second set of contacts for initiating operation of said shiftingmeans.

21. In a speed regulating system for rotary bodies, a constant frequencycurrent interrupter for controlling the speed of the body at relativelyfrequent intervals, a second constant frequently current interrupter forapplying a secondary correction to the speed of said body at relativelylong intervals with respect to said first named speed corrections, avisible indicator of speed rotating with the said body, and meanscontrolled from the first of said named constant frequency vibratingdevices for periodically illuminating said visible indicator whereby dueto the rotation of said indicator with said body the relative speedthereof with respect to the rate of vibration of said first namedconstant period vibrating device may be stroboscopically observed.

22. In combination, a rotating body, a constant period vibrating devicefor controlling the speed of rotation of said body, a visible indicatorattached to and rotatin with said rotating body, and means control ed bsaid constant period vibrating device for i luminating said visibleindicator a predetermined number of times during each rotation of saidrotary body whereby the relative speed of said rotary body with respectto the controlling force thereon may be stroboscopically observed byobserving the relative position at which illumination of said indicatoroccurs at successive rotations of said body.

23. A system for stroboscopically indicating the speed of rotatingbodies and the like, which includes, a rotating body, a constant periodvibrating device for controlling the speed of said body at each rotationthereof, a glow lamp rotating with said body, and means controlled bysaid constant frequency vibrating device for illuminating said glow lamponce during each rotation of said body whereby deviations in the speedof rotation of said body relative to the constant frequency vibratingcontrol device may be ob- Ill served stroboscopically by determiningrelative changes in the position of the point of illumination of saidglow lamp for successive rotations of said body.

5 24. In combination, a rotating body, a constant period vibratory meansfor controlling the speed of rotation of 'said body, a light sourceattached to and rotating with said rotatin body and serving as a visibleindi- 10 cator o? synchronism, and means for illuminating said lightsource a predetermined number of times during each rotation of saidrotary body whereby the relative deviation in speed of said body withrespect to the nor- 15 mal controlling speed may be stroboscopicallyobserved.

' RICHARD H. RANGER.

CERTIFICATE OF CORRECTION.

Patent No. 1,829,420. October 27, 1931.

RICHARD ROWLAND RANGER.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,line 54,for "turning" read tuning; page 6, line 91, claim 21, for"frequently" read frequency; and that the said Letters Patent should beread with these corrections therein that the same may conform to therecord of the case in the Patent Office.

Signed and sealed this 5th day of July, A. D. 1932.

M. J. Moore,

(Seal) Acting Commissioner of Patents.

